Mineral Formation on Metallic Copper in Afuture Repository Site Environment
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SKI Report 96:38 Mineral Formation on Metallic Copper in a "Future Repository Site Environment" Örjan Amcoff Katalin Holényi April 1996 ISSN 1104-1374 ISRN SKI-R--96/38--SE ffiMM^ STATENS KÄRNKRAFTINSPEKTION VOL 2 7 fia 1 5 Swedish Nuclear Power Inspectorate SKI Report 96:38 Mineral Formation on Metallic Copper in a "Future Repository Site Environment" Örjan Amcoff Katalin Holényi University of Uppsala Institute of Earth Sciences, Mineralogy-Petrology, Norbyvägen 18B, 752 36 Uppsala, Sweden April 1996 This report concerns a study which has been conducted for the Swedish Nuclear Power Inspectorate (SKI). The conclusions and viewpoints presented in the report are those of the authors and do not necessarily coincide with those of the SKI. NORSTEDTS TRYCKERI AB Stockholm 1996 SUMMARY. Formation of copper minerals in a "future repository site environment" is discussed. Since reducing conditions are expected much effort has been concentrated on Cu-sulphides and CuFe-sulphides. However, oxidizing conditions are also discussed. A list of copper minerals are included. It is concluded that mineral formation and mineral transitions on the copper canister surface will be governed by kinetics and metastabilities rather than by stability relations. The sulphides formed are less likely to form a passivating layer, and the rate of sulphide growth will probably be governed by the rate of transport of reacting species to the cansiter surface. A series of tests are recommended, in a milieu resembling the initial repository site conditions. SAMMANFATTNING. Bildning av kopparmineral i en framtida slutförvarsmiljö diskuteras. Eftersom reducerande betingelser kan förväntas har Cu-sulfider och CuFe-sulfider getts stor plats, men även oxiderande betingelser diskuteras. En lista på kopparmineral är inkluderad. Slutsatsen är att mineralbildning och mineralförändring på ytan av kopparkapslarna kommer att styras av kinetik och metastabiliteter snarare än av jämviktsrelationer. Det är mindre troligt att sulfiderna kommer att bilda ett passiviserande skikt och sulfideringshastigheten kommer troligtvis att bestämmas av hur fort reagerande ämnen kan transporteras till kapselytan. En serie tester rekommenderas, i en miljö som liknar den initiala slutförvarsmiljön. TABLE OF CONTENTS. 1 INTRODUCTION 1 2 FORMATION OF MINERAL PHASES ON METALLIC COPPER 3 2.1 General 3 2.2 The Cu-Fe-S system 5 2.3 "Problems" concerning low temperature relations in the Cu-Fe-S system 7 2.4 Mechanisms of sulphide formation on metallic copper 13 2.5 "Oxide" formation on metallic copper 1 6 2.6 What phase(s) will form on the canister surface? 20 2.6.1 High sulphur activity conditions 23 2.6.2 Low sulphur activity conditions 24 2.7 Rate of sulphidation on metallic copper 25 3 CONCLUSIONS AND RECOMMENDATIONS 2 8 4 TABLE 1 (copper minerals) 31 5 REFERENCES 37 6 APPENDIX 45 6.1 General 45 6.2 Persistency field Eh-pH relations 49 7 FIGURE CAPTIONS 56 1 INTRODUCTION. The present report was initiated by the Swedish Nuclear Power Inspectorate. It may be regarded as a continuation of our previous report; "Stability of metallic copper in the near surface environment" (SKN 57, 1992), which dealt with geochemical aspects of copper solubility and stability in an expected future "repository site environment for spent nuclear fuel". Here we discuss possible mineral forming processes on the surface of the copper canisters, based on the geological - geochemical literature. The advantage in using geological analogies is that nature provides us with long-term low temperature experiments on mineral behaviour, which we can never hope to copy in the laboratory. The disadvantage is that the conditions, in terms of chemical composition etc., are often ill defined. Unfortunately, information which has a direct bearing on the present well-defined problem is missing, to a large degree. This means that we have to use circumstantial evidence and analogy reasoning, to be able to say anything about possible futures for the canisters. Partly, this problem stems from the fact that studies on ore formation - ore alteration etc., are based on stability relations of various ore minerals at elevated temperatures. Here we are confronted with the problem of predicting what to expect, in an environment (about +5 to +80°C) where most ore minerals fail to equilibrate in the laboratory within reasonable times (This does not mean that the minerals fail to react at all, rather that reactions tend to follow metastable paths at low temperatures. This is true both for natural long-term - and laboratory short- term experiments, and is discussed below). Thus, studies on mineral equilibria are of limited value, since we have to extrapolate experimental results from, say 200 - 300°C, to the present environment. To add to the uncertainty, it is evident that even equilibrium Eh-pH studies, which are frequently employed to delineate mineral stabilities in the natural low-temperature environment, often show large discrepancies when confronted with reality. As exemplified in Appendix 1, Eh-pH values calculated from thermochemical data, often differ significantly from what is actually observed employing potentiometric measurements. Since considerable uncertainty exists as to which minerals (if any!) will actually form on the copper canisters, we have listed both likely and less likely candidates in Table 1. Much of the discussion here is concerned with Cu-sulphides and CuFe- sulpjiides. This is partly so because we are more familiar with these phases compared with Cu-oxides and complex Cu(n)-compounds like malachite; Cu2(CO3)(OH)2 etc., found in environments suggesting oxidizing conditions. Although measurments on deep groundwaters indicate reducing conditions, it is also of interest to analyse the behaviour of copper under oxidizing conditions. Oxidizing conditions will prevail in the repository for some time due to residual oxygen from entrapped air. Formation and alteration of metal sulphides in nature is a question of redox processes. Some basic definitions and basic thermodynamic quantities were given in SKN 57, 1992, as well as a few examples on how to construct a Eh-pH stability diagram. This is not repeated here. Besides, in the present report we discuss qualitative rather than quantitative aspects of mineral formation on the copper canisters. Quantitative aspects depend on the environment, in terms of exact chemical composition of the groundwater and behaviour of the other barriers (bentonite, enclosing rocks). These questions are outside the scope of this report. Much of the text below is given to exemplifications and discussion of low temperature processes observed in nature. As stated above, several of these examples are not directly applicable to the present problem. We think they merit our attention, however, since low temperature processes among metal sulphides show some common behaviour, which can be applied to the question of mineral formation on metallic copper. Also, they illustrate the complexity of the processes to be expected, as well as the difficulty confronting us when we try to make predictions about something we have little solid knowledge about. 2 FORMATION OF MINERAL PHASES ON METALLIC COPPER. 2.1 General. As discussed by Amcoff and Holényi in SKN 57, 1992, there are two major environments in nature where metallic copper is found; (i) In meteorites, "Moon- rocks", and mantle rocks (magmatic rocks with a low SiO2 content, crystallized at great depth), which suggest the strongly reducing environment when the Earth was formed, (ii) In the deeper parts of the oxidized zone of some Cu-rich sulphide ores (still in the upper part of the crust), where conditions are not oxidizing enough for formation of Cu-oxides etc., and not reducing enough for Cu-sulphide formation. It was also suggested that environment (ii) is more relevant when discussing the future repository site environment. As far as we know, geochemical studies concerning the present problem are practically non-existent. This is so, because the interest has been focused more on the particulars of metallic copper formation, in terms of the physico-chemical milieau, than on its subsequent surface alteration characteristics. The reason for this is partly that metallic copper is a rare mineral, its very existence has been considered the most interesting discussion topic. Besides, microscale surface processes have earlier been difficult to study, they have consequently recieved little attention. Thus, information which is of interest in the present context is of a rhapsodic and indirect character. As seen in Table 1, there exist many Cu-minerals in the low temperature environment. Some of these may be of importance here. A number of factors influence mineral formation in the present environment. These include; (i) Temperature and pressure, (ii) Chemical environment in terms of aqueous and solid species which may participate in reactions with the metal, (iii) Stability-metastability. Again we would like to stress that the present report is concerned with qualitative aspects of mineral formation. (For example, the redox conditions in terms of Eh tell us little about the redox capacity, which depends on how fast redox couples which participate in reactions are replenished. This quantity is of fundamental importance when it comes to the question of long-term survival of the copper canisters). For the sake of argument we assume that the groundwater surrounding the canisters contains a certain amount of species with the ability to oxidize metallic copper; that some sulphur is present in the form of aqueous sulphides and polysulphides (discussed below), and that some dissolved iron is present (SKB 86-03, 87-07). Also we assume a temperature below 100°C and a pH slightly on the alkaline side of neutrality. Here we will not discuss the possibility that the copper canisters might accidentally be subjected to strong oxidation "from above". A sulphidation process is equivalent with a redox process, independant of whether it takes place in an aqueous or a dry environment. Copper in binary Cu-sulphides seems to be univalent, independent of the Cu/S ratio (Rolf Berger, Institute of Chemistry, Uppsala University, personal communications).